A variety of treatments such as coating, laminating or the like are applied to metal sheets or plated metal sheets which are to be used for various purposes so that those metal sheets can have characteristics such as proper appearance, corrosion resistance and/or insulation. For this purpose, chromate treatment may be applied on the surface of the metal sheet for substrate-treating. The chromate treatment is made for improving the corrosion resistance and the adhesiveness between the metal sheet and the resin layer such as a coated layer and/or laminate formed on the metal sheet.
In view of the recent growing concern of the environment, however, it has been desired not to use a chromate treatment containing hexavalent chromium which has a large negative environmental impact. Under the circumstance, chromate-free surface treatment has been developed in order to have the same performance as the conventional chromate treatment, yet the performance of the chromate-free treatment needs great improvement.
For example JP-A2000-282256 discloses a treatment agent as a corrosion inhibitor for aluminum containing a compound made of zinc compound and zirconium compound selected from the group consisting of an oxide, oxyacid salt, organic acid salt, fluorocomplex salt, of zirconium or mixtures thereof. It further discloses that a coated film of a zirconium compound containing a zinc compound is formed by applying the above-mentioned treatment agent onto an aluminum substrate using coating, dipping, spraying or the like and then is dried, which leads to improvement in corrosion resistance. Although it is required for the alternative method to have an excellent adhesiveness with the resin formed on the substrate as well as corrosion resistance, the reference does not mention resin-adhesiveness, i.e., the adhesiveness between the coated film of the zirconium compound and the coating or resin laminate formed on the coated film.
JP-A2000-282267 discloses a treatment agent as a corrosion inhibitor for aluminum wherein the treatment agent contains a hydrophilic resin, a crosslinkable resin reactable with zirconium and at least one selected from the group consisting of an oxide, oxyacid salt, organic acid salt or fluorocomplex salt of zirconium. It further discloses that a coated film including zirconium compound and a hydrophilic organic resin is formed by applying the treatment agent onto an aluminum substrate of a heat exchanger by coating, dipping, spraying or the like and which is then dried, which imparts hydrophilicity as well as corrosion resistance. Since the hydrophilicity imparted to the coated film is utilized, the aluminum material with the coated film can be directly used for the heat exchanger without applying a coating or laminate thereon. Thus, there is no mention of resin-adhesiveness in the case where a coating and/or resin laminate are applied on the coated film.
Further, another method of chromate-free treatment is disclosed in JP-A2002-30460. This reference proposes a metal surface treatment agent containing a vanadium compound as an essential component and an organic compound with a functional group. It further discloses that a coated film of a vanadium compound (where pentavalent vanadium (V(V)) which is poor in water resistance and alkali resistance has been reduced to tetravalent (V(IV)), trivalent (V(III)) and/or bivalent vanadium (V(II))) is formed by applying the treatment agent onto the surface of a metal material using roll-coating, dipping or the like and then is dried, which leads to improvement in corrosion resistance. The reason for the addition of the organic compound is to stabilize a vanadium compound in a treatment liquid by chelating vanadium reduced from pentavalent vanadium (V(V)) to tetravalent (V(IV)), trivalent (V(III)) and/or bivalent vanadium (V(II)). The reference also discloses the addition of water-soluble polymer to the processing liquid and formation of an additional organic polymer film on top of the coated film to improve corrosion resistance, fingerprinting resistance and surface lubricating ability. However there is no mention of adhesion property to resin in the case of application of the coating and/or resin laminate on the coated film.
As mentioned above, conventional chromate-free treatment is a method which is performed to form a coated film by applying a treatment liquid and drying it, if desired, using a heat-drying or baking treatment. That is, JP-A2000-282256 and JP-A2000-282267 teach that cross-linking is completed by baking and JP-A2002-30460 teaches both the application and drying of the treatment, which gives improved film forming ability and adhesiveness by performing heat-drying. Such an application type film forming method is a simple process similar to chromate treatment. However, in the above-mentioned chromate-free coated film prepared for its barrier function, if the coated film is not entirely formed on the substrate, i.e., a specific part of the coated film is not adhered to the substrate, corrosion will start in the specific part. Further, the conventional method needs a coating matrix or binder such as a resin as in the pigment of JP-A07-278853, which leads to the lowering of the percentage of inorganic components and may result in insufficient performance. It is possible to increase the percentage of the inorganic components by using a sol-gel method. However, this requires that the object is heated up to the temperature of several hundred degrees Celsius, which makes it impossible to contain an organic component for improving the resin-adhesiveness. (See FIG. 1)
An electrochemical precipitation method, such as the one shown in WO2003/048416, has been developed as a method for providing a coated film at room temperature. The coated film is easily formed over the entire substrate, with less defects, having a high inorganic component percentage, a high density and a high barrier function. This method includes not only merely dipping the substrate in the treatment liquid, but also forming an oxide film quickly by performing cathode electrolyzation by applying an electric voltage between the substrate and the counter electrode. The reaction formula with respect to this method is described below taking zirconia as an example.ZrF62−+2H2OZrO2+4H++6F−In the equilibrium reaction, it was found that zirconia is formed when hydrogen ion and fluorine ion are consumed. The zirconia formation rate can be controlled if electrolyzation is used for consuming the ions of hydrogen and fluorine. The coated film, formed using the above-described method, is dense and has little defects, which provides an excellent corrosion resistance compared to the conventional coated film, and is equivalent to or superior to that provided by conventional chromate treatment. However, since another important effect resulting from chromate treatment, i.e., good resin-adhesiveness, has not been developed yet, urgent development has been desired.
As described above, with respect to a coated film formed by the chromate-free treatment method, the adhesiveness between the coated film and the resin layer formed on the coated film (resin-adhesiveness) has not been sufficiently studied and a coated film providing excellent resin adhesiveness has not been obtained yet. For example, according to an investigation by the inventors, it was found that the coated film described in JP-A2000-282256 does not provide sufficient resin adhesiveness under wet conditions. As for the coated film described in JP-A2000-282267, a strong resin layer is expected because the coated film contains a cross-linkable resin to compensate for the poor water-resistance of the hydrophilic resin in order to provide a good film-forming property. However, since it lowers the percent content of zirconium as an inorganic component which provides a barrier function, the corrosion resistance becomes insufficient. As for the coated film described in JP-A2002-50460, high resin-adhesiveness is seemingly expected because the organic resin is added to a vanadium compound. However since fingerprinting resistance is improved by the resin addition, adhesiveness to the organic material such as the resin is lowered. Although a reduced vanadium compound is normally capable of improving corrosion resistance by delocalized corrosion electrons because of its electrical conductivity, the delocalized effect is reduced when the organic resin is mixed, which leads to insufficient resin adhesiveness. As for the coated film described in WO2003/048416, although the corrosion resistance is extremely excellent, since almost no defect can be created at the formation of the coated film, the resin adhesiveness is insufficient in the case of applying a resin layer such as a coating or laminate on the coated film.